In order to provide higher data rate transmissions and to support various applications, telecommunication service providers continually develop improvements in existing networks. Wider bandwidth allocation is a way to achieve the targets. However, it is difficult to assign a wide range contiguous spectrum for an access network due to existing operations on certain spectra. Accordingly, it is expected that broadband wireless access networks of next generation can be deployed by using a combination of different spectra. Consequently, technologies supporting a combination/concatenation of channel bandwidths to best utilize the available spectrum should be developed to enable data transmissions over multiple carriers. With the application of a scheme referred to as Carrier Aggregation or Multiple “Component Carriers” (CCs), networks can be enabled to operate over continuous or discontinuous carriers having different bandwidths.
The carrier aggregation scheme may utilize separate FFT and radio frequency (RF) modules for each individual band. Based on user capabilities, network entities (e.g. a base station (BS), a Node-B, an eNode-B, a base transceiver system (BTS), an access point (AP), a home base station, a relay station, a scatterer, a repeater, an intermediate node, an intermediary, and/or satellite-based communication base station, etc.) can serve different users with corresponding different bandwidths. With the support of multiple CCs, a BS can flexibly use limited bandwidth to achieve high throughput to improve user experience at the user equipment (UE).
In Carrier Aggregation (CA), two or more component carriers (CCs) or cells can be aggregated in order to support wider transmission bandwidths. A UE can simultaneously receive or transmit one or multiple CCs depending on its capabilities. A UE may be configured with more than one Cell. One of them is Primary Cell (PCell), and the other(s) is/are secondary cell(s) (SCell).
For example, FIG. 1 illustrates a conventional LTE TDD UL-DL configuration table. As shown in FIG. 1, there are 7 UL/DL configurations supported in LTE TDD systems, in which a LTE TDD system with TDD DL/UL subframe configuration#0 implies that Subframes 0 and 5 are DL subframes; Subframes 1 and 6 are SPECIAL subframes; and Subframe 2, 3, 4, 7, 8, 9 are UL subframes.
FIG. 2 illustrates a DL ACK/NACK Timing in TDD. For PUSCH transmissions scheduled from a serving cell c in subframe n, a UE shall determine the corresponding PHICH resource of serving cell c for receiving ACK/NACK feedbacks for PUSCH transmissions in subframe n+kPHICH, where kPHICH is given in the table of FIG. 2.
On the other hand, cross-carrier scheduling is introduced for carrier aggregation in LTE Rel-10. FIG. 3 is a schematic diagram illustrating cross-carrier scheduling. Referring to FIG. 3, cross-carrier scheduling is configured to allow the control signalling (PDCCH) of a serving cell to schedule resources on another serving cell (e.g., The PDCCH of the CC#2 schedules resources for CC#3) in order to reduce PDCCH channel interference. In this example, CC#2 is the scheduling cell of the CC#3, and the CC#3 is called scheduled cell. Carrier indicator field (CIF) included in the PDCCH of the scheduling cell indicates the cell identity (e.g., CellIndex) of the scheduled cell. When the PDCCH of a SCell is configured, cross-carrier scheduling does not apply to this SCell since this SCell is always scheduled via its PDCCH. However, when a CA-capable UE is configured with two cells with different TDD UL-DL configuration, the following issues may need to take into consideration.
The first issue is: no PDCCH on the Scheduling Cell for DL or UL resource assignments. To be specific, when a CA-capable UE is configured with two cells with different TDD UL-DL configuration, some DL resources on the scheduled cell can not be allocated to the UE, because there is no PDCCH on the scheduling cell for cross-carrier scheduling. FIG. 4 illustrates an example of no PDCCH on the Scheduling Cell for DL and UL resource assignments. In this example, the scheduling Cell of the Cell#3 for a UE is the PCell (Cell#0). In other words, the DL resource of the Cell#3 at the subframe#4 shall be allocated by the subframe #4 of the PCell. However, PCell's subframe#4 is a UL subframe and there is no available DL resource that can be used for cross-carrier scheduling. Thus, the subframe#4 on Cell#3 can't be used by the UE, and the UE can't fully use the bandwidth of the aggregated Cell#3.
The second issue is on HARQ ACK/NACK feedback timing. FIG. 5 illustrates an example of HARQ ACK/NACK feedback timing. As illustrated in FIG. 5, based on HARQ ACK/NACK timing (e.g., illustrated in FIG. 2), the corresponding subframe on the scheduling cell to reply HARQ ACK/NACK for the previous PUSCH transmitted on the scheduled cell is an UL subframe. In other words, there is no DL resource to send the HARQ ACK/NACK. Thus, the UE needs to re-transmit the UL data again, because no ACK is received.
Accordingly, there is a need to develop a new scheme to provide downlink control signalling for the UE, which is configured with multiple TDD cells with different UL-DL configurations.